Aluminum alloys having high corrosion resistance after brazing

ABSTRACT

When using AA3000 series and AA1000 series aluminum alloys to produce extruded products for heat exchanger applications, by controlling the level of copper and nickel in the alloy to very low levels it is possible to produce excellent corrosion resistance both before and after a brazing cycle. To achieve these results, the copper content should be no more than 0.006% by weight and the nickel no more than 0.005% by weight. A typical alloy of the invention contains about 0.001-0.5% by weight manganese, 0.001-0.7% by weight iron, 0.001-0.02% by weight titanium, 0.001-0.3% by weight silicon, less than 0.006% by weight copper, less than 0.005% by weight nickel and 0.001-0.02% by weight zinc, with the balance consisting of aluminum and incidental impurities. No zinc addition to the alloy is required either by zinc spraying or by alloy addition.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority right of provisional applicationSer. No. 60/187,835 filed Mar. 8, 2000 by applicants herein.

TECHNICAL FIELD

This invention relates to corrosion resistant aluminum alloys and, moreparticularly, to an AA3000 series type aluminum alloy having low levelsof copper and nickel and showing excellent corrosion resistance afterbrazing.

BACKGROUND ART

Aluminum alloys are well recognized for their corrosion resistance. Inthe automotive industry, aluminum alloys are used extensively for tubingbecause of their extrudability as well as the combination of lightweight and high strength. They are used particularly for heat exchangeror air conditioning condenser applications, where excellent strength,corrosion resistance and extrudability are necessary.

The AA1000 series aluminum alloys are often selected where corrosionresistance is needed. Where higher strengths are required, the AA3000series aluminum alloys are often used.

A difficulty with use of aluminum alloys in corrosive environments, suchas heat exchanger tubing, is pitting corrosion. There are variousmethods currently in use for protecting tubing manufactured from cladsheet stock from corrosion. One of these consists of using a core alloye.g. X800 or X900, which protects from corrosion through the formationof a dense precipitate band within the core adjacent to the cladding.This band corrodes preferentially to the rest of the core therebyincreasing the life of the tube. This method is only applicable to sheetproducts due to the need for a clad layer of Al—Si alloy on the tubesurface.

Alloys currently in use for brazed extruded tubing do not give goodcorrosion resistance unless the tubing is sprayed with zinc prior tobrazing. This is typically carried out in-line at the extrusion pressand is costly and undesirable from an environmental viewpoint.

Sircar, WO 97/46726, published Dec. 11, 1997 describes a corrosionresistant AA3000 series aluminum alloy containing low copper, but highzinc and titanium. While this alloy provides reduced corrosion, itstitanium content of 0.03 to 0.30% by weight raises the pressuresrequired for extrusion, thereby lowering productivity.

U.S. Pat. No. 5,286,316 (Wade) describes an alloy consisting essentiallyof 0.1-0.29% by weight manganese, 0.05-0.12% by weight silicon,0.10-0.20% by weight titanium, 0.15-0.25% by weight iron with thebalance aluminum. This alloy contained very low levels of copper notexceeding 0.03% by weight, but again a quite high titanium content.

In Auran et al, EP 0899350, published Mar. 3, 1999, a corrosionresistant alloy is described containing 0.10-0.40% by weight iron,0.05-0.25% by weight silicon, 0.12-0.22% by weight titanium, less than0.10% by weight manganese, less than 0.35% by weight copper and thebalance aluminum.

In Auran et all WO 99/04051, published Jan. 28, 1999, another corrosionresistant alloy is described preferably consisting of 0.06-0.15% byweight iron, 0.05-0.15% by weight silicon, 0.03-0.08% by weightmanganese, 0.10-0.18% by weight titanium, 0.10-0.18% by weight chromium,less than 0.01% by weight copper, 0.10-0.18% by weight zinc and thebalance aluminum.

Jeffrey et al., CA 2,247,037 teaches extruding aluminum alloys havinggenerally high zinc and low titanium into heat exchanges tubing.Corrosion testing was conducted on tubing which had not been exposed tofurnace brazing conditions. An alloy with a low zinc content (0.0022%)failed the corrosion test.

Other references include Hufnagel W., “Key to Aluminum Alloys, 4thEdition” 1992 showing typical compositions for 1000 and 3000 seriesaluminum alloys.

U.S. Pat. No. 5,976,278 describes an aluminum alloy containing up to0.03% copper, 0.1-1.5% manganese, 0.03-0.35% titanium, up to 1.0%magnesium, up to 0.01% nickel, 0.06-1.0% zinc, up to 0.3% zirconium, upto 0.50% iron and silicon and up to 0.20% chromium.

An aluminum alloy is described in U.S. Pat. No. 5,906,689 containing upto 0.03% copper, 0.1-0.5% manganese, 0.03-0.30% titanium, 0.06-1.0%zinc, up to 0.50% iron, 0.05-0.12% silicon, up to 0.01% manganese, up to0.01% nickel and up to 0.5% chromium.

In U.S. Pat. No. 5,286,316, an aluminum alloy is described containing0.1-0.5% manganese, 0.05-0.12% silicon, 0.10-0.20% titanium and0.15-0.25% iron.

JP-A-2000-119784 describes a further aluminum alloy containing manganesealong with magnesium, copper, silicon, zinc, nickel and iron.

It is an object of this invention to produce an aluminum alloy of theAA3000 type having excellent corrosion resistance when extruded intotubing and brazed without the need to add zinc by either zinc sprayingor by alloy addition.

DISCLOSURE OF THE INVENTION

It has been found according to the present invention that when usingAA3000 series aluminum alloys to produce extruded products for heatexchanger applications, by controlling the level of copper and nickel inthe alloy to very low levels it is possible to produce excellentcorrosion resistance both before and after a brazing cycle. To achievethese results, the copper content of the alloy is no greater than 0.006%by weight and the nickel content is no greater than 0.005% by weight.These alloys typically contain up to about 1.5% by weight manganese, upto about 0.70% by weight iron, up to about 0.02% by weight titanium, upto about 0.30% by weight silicon, less than about 0.03% by weight zincand the minimum copper and nickel contents as stated above. The balanceconsists of aluminum and incidental impurities.

According to a preferred embodiment, the alloy of the extruded productof the invention is an aluminum alloy containing about 0.001-0.5% byweight manganese, 0.001-0.7% by weight iron, 0.001-0.02% by weighttitanium, 0.001-0.3% by weight silicon, less than 0.006% by weightcopper, less than 0.005% by weight nickel and 0.001-0.02% by weightzinc, with the balance consisting of aluminum and incidental impurities.

According to another preferred embodiment, the alloy of the extrudedproduct of the invention contains less than 0.03% by weight zinc and hasa corrosion resistance sufficient to pass a 20 day SWAAT corrosion test.

Although individual aluminum smelters may already exist that producealuminum containing the above low levels of copper and nickel, the valueof having a product with such low levels was not previously recognized.Accordingly, no effort has been made to isolate ingots during aluminumproduction such that a population of commercial aluminum ingots whenre-heated for extruding into tubing will provide a melt containing lessthan 0.006% copper and less than 0.005% nickel. It is a feature of thisinvention that a population of aluminum alloy ingots used for extrusioninto tubing will on average contain less than 0.006% copper and lessthan 0.005% nickel. Thus, a population of ingots is selected whereby theaggregate population has the above composition.

A typical alloy used in the present invention not only has very lowlevels of copper and nickel but may also be used without any deliberateadditions of titanium or zinc. Thus, the extruded product may be devoidof any surface coating of zinc. The low level of titanium present istypically that remaining from grain refiner addition.

When these alloys are extruded into heat exchanger tubing, andespecially micro-port tubing, they produce sufficient corrosionresistance on their own, thereby eliminating any need for thetraditional zinc thermo-spraying step. However, when these alloys arecombined with a zinc spraying step the corrosion performance will befurther improved. It is a particularly significant feature of thesealloys containing very low levels of copper and nickel that they can beextruded into tubing and processed through a vacuum or inert atmospherebrazing cycle, while still exhibiting excellent corrosion resistance.These alloys also give excellent corrosion resistance for tubing used inmechanically assembled heat exchangers where no brazing cycle is used.

The extruded products of these inventions having high corrosionresistance are typically produced by the following steps:

-   a) casting an ingot of an aluminum alloy as described above;-   b) homogenizing the ingot at a temperature between about 400° C. and    about 650° C.;-   c) cooling the ingot to ambient temperature;-   d) re-heating the ingot and extruding into tubing.

When the tubing is subjected to a brazing cycle, this is done either ina vacuum or an inert atmosphere. For the brazing, the tubing istypically heated at a rate of about 5 to 30° C./min up to a temperatureof about 585 to 615° C. followed by rapid cooling.

BEST MODE FOR CARYING OUT THE INVENTION EXAMPLE 1

A series of alloy compositions based on AA3102 aluminum alloy were D.C.cast as 4″ diameter ingots. These had fixed iron, silicon and manganesecontents but contained incremental additions of copper and nickel. Thetitanium was present only from grain refiner addition and was not adeliberate alloy addition. There was no deliberate addition of zinc anda background level of 0.005/0.006 wt % was present in the aluminum usedto make up the melt. Also included was a commercially produced AA3102alloy. The latter was cast as a 6″ diameter ingot.

These alloy compositions are shown in Table 1 below:

TABLE 1 Commer- Cast H309 H310 H311 J370 J371 1372 cial 3102 Si 0.080.08 0.08 0.08 0.08 0.08 0.08 Fe 0.44 0.45 0.44 0.44 0.44 0.45 0.44 Cu0.001 <0.001 0.001 0.006 0.013 0.022 <0.02 Mn 0.23 0.23 0.23 0.23 0.230.23 0.24 Zn 0.005 0.005 0.005 0.006 0.006 0.006 <0.02 Ni 0.001 0.0020.0052 0.0009 0.001 0.001 <0.01 Ti 0.008 0.01 0.007 0.008 0.007 0.007Brazed 0/5 0/5 2/5 4/5 5/5 5/5 5/5 Per- formance As 0/5 0/5 0/5 0/5 5/55/5 5/5 Extruded Per- formance

The ingots were homogenized for 4 hrs. at 620° C. and cooled at 150°C./hr to room temperature. The metal was extruded into a 0.25 inchdiameter round tube having a wall thickness of 0.016 inch using normalextrusion conditions and the product was air cooled to room temperature.The tubing was cut into 8 inch lengths. Five lengths of each alloy weregiven an inert atmosphere brazing cycle consisting of 20° C./min up to625° C. followed by fast cooling to room temperature, and five were keptin the as-extruded condition. The tubes were then exposed to a corrosiveenvironment in a SWAAT cabinet according to ASTM spec #G85—Annex A3.After twenty days exposure the tubes were removed and checked forperforation using compressed air at a pressure of 80 psi. The tableabove lists the number out of five that perforated for the brazed andnon brazed conditions. In the non brazed condition no tubes failedproviding the copper content was 0.006 wt % or less. After a brazingcycle no tubes failed providing the copper content was less than 0.006wt % and nickel was less than 0.005 wt %. This shows that a long postbrazed life can be achieved in a regular AA3102 Al—Mn type alloyextrusion if copper and nickel impurity levels are maintained belowcritical values (0.006 and 0.005 wt % respectively).

EXAMPLE 2

A further series of four AA3102 type Al—Mn alloy compositions were D.C.cast as 6″ diameter billets. The aluminum used to make up the melts wasselected to have a copper content of 0.0012-0.0015 wt %. The nickelcontent was 0.0015 to 0.0019 wt %. The base level of zinc in thealuminum was 0.0021-0.0025 wt %. The four casts were alloyed so as toproduce the following variants:

Low iron low zinc

Low iron high zinc

High iron low zinc

High iron high zinc

The compositions are shown in Table 2 below:

TABLE 2 Cast Si Fe Cu Mn Zn Ni Ti MNL 0.072 0.077 0.0012 0.21 0.00210.0015 0.0048 MNM 0.07 0.076 0.0012 0.23 0.0225 0.0015 0.0048 MNO 0.0610.44 0.0015 0.23 0.0025 0.0018 0.0044 MNP 0.064 0.45 0.0015 0.23 0.02130.0019 0.0045

Silicon levels were maintained at ˜0.07 wt % and Mn was maintained at˜0.23 wt % for all the casts. The titanium content was due to theaddition of Ti/B grain refiner used to control the cast grain size andwas not a deliberate alloy addition. The billets were homogenized usingthe same procedure as in Example 1 and were extruded into a 0.25inch×0.016 inch round tube. The tubing was cut into 8 inch lengths andgiven a simulated vacuum brazing cycle. The cycle consisted of a 25°C./min heat up to 500° C. followed by 15° C./min to 600° C. then 5°C./min to 615° C. The samples actually received about 2 min soak between600° C. and 607° C. then were cooled in the furnace until 500° C. (about5 minutes) before being removed from furnace. A vacuum at about 5×10⁻⁵Torr was used at brazing temperature.

The samples were then corrosion tested using SWAAT test as described inExample 1 and samples were taken out after 20 (2 tubes each), 25 (2tubes each), 30 (2 tubes each) and 40 days (6 tubes each). The tubeswere tested for perforation using compressed air at 80 psi. Table 3below summarized the results:

TABLE 3 Cast 20 days 25 days 30 days 40 days MNL OK OK OK 5 out of 6 OKMNM OK OK 1 out of 2 OK 4 out of 6 perf. MNO OK OK OK 5 out of 6 OK MNPOK OK 1 out of 2 OK All perf.

All of the tubes passed a 20-day SWAAT test, which was the criterion inExample 1, and also passed 25 days exposure. This supports theconclusion of Example 1 that an alloy with Ni less than 0.005 and Culess than 0.006 wt % can pass a 20 day SWAAT post brazed. After thirtydays exposure, failures were observed in the alloys containing thehigher level of zinc. After 40 days exposure some perforations wereencountered for all the alloys but the performance of the compositionswith the higher zinc level was noticeably inferior. At the lower zinclevel there was no measurable effect associated with changing the ironlevel from 0.07 to 0.44 and both alloys performed in an equivalentmanner. This shows that maintaining low copper (<0.006 wt %) and nickel(<0.005 wt %) gives 25 days SWAAT life post brazed for iron levels of0.07 to 0.44 wt % and zinc levels of 0.002 to 0.025 wt %. Superiorperformance can be achieved (up to 30 days—no perforation) if the zinccontent is kept <0.025 wt %.

EXAMPLE 3

A series of three alloys were cast, homogenized and extruded into 0.014inch×0.016 inch tubing in the same manner as in Example 2. Acommercially produced AA3102 ingot was also included The alloys had thecompositions shown in Table 4 below:

TABLE 4 Cast Si Fe Cu Mn Zn Ni Ti MKX 0.08 0.08 0.0016 0.24 0.17 0.00220.0011 MKY 0.08 0.41 0.0017 0.24 0.17 0.0024 0.01 MGM 0.07 0.07 0.00220.24 0.17 0.0012 0.14 Production 0.08 0.39 0.0179 0.21 0.014 0.0047 0.013102

A total of ten 8 inch lengths of tubing were produced for each alloy.Half of these were given a 6% stretch to simulate the cold workassociated with coiling/uncoiling and straightening operations, whichwould occur in an industrial situation. The tubes were then exposed to avacuum brazing cycle as in Example 2 before being SWAAT tested for 20days.

All the tubes failed a 20 day SWAAT test by perforation as tested bycompressed air.

In terms of composition the three experimental alloys all had low copperand nickel levels but had a deliberate zinc addition of 0.17 wt %. AlloyMGM also contained an increased Ti addition. The production alloycontained low zinc but contained copper >0.006 wt %. The results showthat for vacuum brazed tubing, an alloy with low copper and nickel alongwith a deliberate zinc addition does not give 20 day SWAAT life.

1. A furnace brazed aluminum alloy extruded product having corrosionresistance wherein the alloy consists essentially of an aluminum alloyof the AA3000 series containing less than 0.006% by weight copper, lessthan 0.005% by weight nickel and less than 0.03% by weight zinc, andwherein the extruded product is devoid of any surface coating of zinc.2. An aluminum alloy extruded product according to claim 1 wherein theextruded product is a beat exchanger tube.
 3. An aluminum alloy extrudedproduct according to claim 1 wherein the alloy contains up to 1.5% byweight manganese, up to 0.70% by weight iron, up to 0.02% by weighttitanium, up to 0.30% by weight silicon, less than 0.006% by weightcopper, less than 0.005% by weight nickel and less than 0.03% by weightzinc, the balance aluminum and incidental impurities.
 4. An aluminumalloy extruded product according to claim 3 wherein the alloy contains0.001-0.5% by weight manganese, 0.001-0.7% by weight iron, 0.001-0.02%by weight titanium, 0.001-0.3% by weight silicon, less than 0.006% byweight copper, less than 0.005% by weight nickel, 0.001-0.02% by weightzinc, the balance aluminum and incidental impurities.
 5. An aluminumalloy extruded product according to claim 1 having a corrosionresistance sufficient to pass a 20 day SWAAT corrosion test.